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Herz Group

Prof Laura Herz FRS

Professor of Physics

Sub department

  • Condensed Matter Physics

Research groups

  • Semiconductors group
  • Advanced Device Concepts for Next-Generation Photovoltaics
Laura.Herz@physics.ox.ac.uk
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Publons/WoS
  • About
  • Publications

Photovoltaic performance of FAPbI3 perovskite is hampered by intrinsic quantum confinement

ACS Energy Letters American Chemical Society 8:6 (2023) 2543-2551

Authors:

Karim A Elmestekawy, Benjamin M Gallant, Adam D Wright, Philippe Holzhey, Nakita K Noel, Michael B Johnston, Henry J Snaith, Laura M Herz

Abstract:

Formamidinium lead trioiodide (FAPbI3) is a promising perovskite for single-junction solar cells. However, FAPbI3 is metastable at room temperature and can cause intrinsic quantum confinement effects apparent through a series of above-bandgap absorption peaks. Here, we explore three common solution-based film-fabrication methods, neat N,N-dimethylformamide (DMF)–dimethyl sulfoxide (DMSO) solvent, DMF-DMSO with methylammonium chloride, and a sequential deposition approach. The latter two offer enhanced nucleation and crystallization control and suppress such quantum confinement effects. We show that elimination of these absorption features yields increased power conversion efficiencies (PCEs) and short-circuit currents, suggesting that quantum confinement hinders charge extraction. A meta-analysis of literature reports, covering 244 articles and 825 photovoltaic devices incorporating FAPbI3 films corroborates our findings, indicating that PCEs rarely exceed a 20% threshold when such absorption features are present. Accordingly, ensuring the absence of these absorption features should be the first assessment when designing fabrication approaches for high-efficiency FAPbI3 solar cells.

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Exciton formation dynamics and band-like free charge-carrier transport in 2D metal halide perovskite semiconductors

Advanced Functional Materials Wiley 33:32 (2023) 2300363

Authors:

Silvia G Motti, Manuel Kober-Czerny, Marcello Righetto, Philippe Holzhey, Joel Smith, Hans Kraus, Henry J Snaith, Michael B Johnston, Laura M Herz

Abstract:

Metal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high-efficiency photovoltaic applications. Low-dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge-carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature-dependent measurements of free charge-carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in-plane mobilities. Enhanced charge-phonon coupling is shown to reduce charge-carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge-carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge-carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature-dependent interplay of exciton and free-carrier populations in 2D MHPs. Furthermore, such sustained free charge-carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.
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Narrowband, angle-tuneable, helicity-dependent terahertz emission from nanowires of the topological Dirac semimetal Cd3As2

ACS Photonics American Chemical Society 10:5 (2023) 1473-1484

Authors:

Chelsea Xia, Dharmalingam Prabhakaran, Laura Herz, Thorsten Hesjedal, Michael Johnston

Abstract:

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarisation control remains difficult. Here, we demonstrate that by exploiting bandstructure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd3As2. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarisation. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21 – 1.40 THz as the angle is tuned from 15° - 45°. We therefore present Cd3As2 nanowires as a promising novel material platform for controllable terahertz emission.
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Temperature dependent reversal of phase segregation in mixed-halide perovskites

Advanced Materials Wiley 35:19 (2023) 2210834

Authors:

Adam D Wright, Jay B Patel, Michael B Johnston, Laura M Herz

Abstract:

Understanding the mechanism of light-induced halide segregation in mixed-halide perovskites is essential for their application in multijunction solar cells. Here, photoluminescence spectroscopy is used to uncover how both increases in temperature and light intensity can counteract the halide segregation process. It is observed that, with increasing temperature, halide segregation in CH3NH3Pb(Br0.4I0.6)3 first accelerates toward ≈290 K, before slowing down again toward higher temperatures. Such reversal is attributed to the trade-off between the temperature activation of segregation, for example through enhanced ionic migration, and its inhibition by entropic factors. High light intensities meanwhile can also reverse halide segregation; however, this is found to be only a transient process that abates on the time scale of minutes. Overall, these observations pave the way for a more complete model of halide segregation and aid the development of highly efficient and stable perovskite multijunction and concentrator photovoltaics.
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Charge-carrier dynamics of solution-processed antimony- and bismuth-based chalcogenide thin films

ACS Energy Letters American Chemical Society 8:3 (2023) 1485-1492

Authors:

Z Jia, M Righetto, Y Yang, Cq Xia, Y Li, R Li, Y Li, B Yu, Y Liu, H Huang, Mb Johnston, Lm Herz, Q Lin

Abstract:

Chalcogenide-based semiconductors have recently emerged as promising candidates for optoelectronic devices, benefiting from their low-cost, solution processability, excellent stability and tunable optoelectronic properties. However, the understanding of their fundamental optoelectronic properties is far behind the success of device performance and starts to limit their further development. To fill this gap, we conduct a comparative study of chalcogenide absorbers across a wide material space, in order to assess their suitability for different types of applications. We utilize optical-pump terahertz-probe spectroscopy and time-resolved microwave conductivity techniques to fully analyze their charge-carrier dynamics. We show that antimony-based chalcogenide thin films exhibit relatively low charge-carrier mobilities and short lifetimes, compared with bismuth-based chalcogenides. In particular, AgBiS2 thin films possess the highest mobility, and Sb2S3 thin films have less energetic disorder, which are beneficial for photovoltaic devices. On the contrary, Bi2S3 showed ultralong carrier lifetime and high photoconductive gain, which is beneficial for designing photoconductors.
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